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STUDY: Sintering Structural Powder Metallurgy Components at 2500°.

Posted by Fran Hanejko - June 25, 2021

“Conventional” sintering is synonymous with structural powder metallurgy components. 

The majority of today’s powder metal parts are sintered at 2050° in a nitrogen-hydrogen atmosphere. This has proven satisfactory for legacy applications, and will remain “good enough” for many future applications. 

With the ongoing “electrification” of transportation, many engineers are leaving the world of “good enough.” 

Their applications demand improved properties, so they’ve made the jump from conventional to high-temperature sintering (2125-2300° F).  

However, the changing dynamics of motors and transportation have forced further rethinking of the performance of powder metal and competing parts. Specifically, newer parts demand higher tensile and fatigue strengths.

Is another jump imminent? Recently, we helped investigate the potential benefits of sintering at temperatures approaching 2500°.  

This “ultra-high-temperature sintering” (UHTS) enhances the many advantages of current high-temperature sintering. The potential exists -- not just in a few years, but today -- to further leverage non-traditional alloys and achieve even greater mechanical properties.

Your design could achieve a strength-ductility combo that’s superior to any current iron-based powder metal material. And powder metallurgy manufacturers now have the tools to take on applications previously thought impossible with PM.

That’s the short version (honestly!) of our investigation’s results. For a deeper dive into unique alloying additions, processing details, and the resulting mechanical property boosts UHTS delivers, keep reading. 

What Temperatures Are Powder Metallurgy Components Sintered At?

First, let’s define what we consider to be:

  • Sintering (good)
  • High-temperature sintering (better)
  • Ultra-high-temperature sintering (best)

Sintering temperature represents more than just a number. It’s a window of opportunity for part performance and customization.

Conventional Sintering in Powder Metallurgy (~2050°)

Proper sintering of the compacted powder metal parts includes:

  • Delubrication
  • Oxide reduction
  • Formation of metallurgical bonds between pressed particles
  • Complete or partial alloying

More than 80% of all structural powder metal components are sintered at 2050°, according to estimates. Indeed, these sintering conditions have proven adequate for a wide variety of powder metal parts currently in production.  

Despite this universal acceptance, limitations exist for sintering in this temperature range:

  • Incomplete homogenization of premixed alloys, such as copper
  • Alloying elements such as manganese, chromium, silicon, and vanadium may not reduce at 2050°
  • Limited availability of premix additives to improve mechanical properties (nickel, silicon, and vanadium)
  • Prealloyed powders containing silicon, chromium, and vanadium are often trickier to compress

 

High-Temperature Sintering (2125-2300°)

Sintering non-stainless ferrous materials at higher temperatures ups the performance noticeably:

  • Higher density
  • Better mechanical properties
  • Can use non-traditional PM alloying elements
  • Reduce time at temperature, partially offsetting the increased cost of high-temperature sintering

A recent study reported that high-temperature sintering improved:

  • Tensile strength
  • Fatigue performance
  • Modulus of elasticity

Despite the many benefits of high-temperature sintering, current market use for this technology is quite low. So, what’s the holdup?

  • An assumed lack of dimensional precision
  • Capital cost of equipment
  • Cost (both fixed and maintenance) associated with the technology

 

Ultra-High-Temperature Sintering (up to 2500°)

Recently, a new furnace design set the bar for sintering.

This ultra-high temperature sintering enhances the many advantages of high-temperature sintering, with expanded mechanical performance and alloying options. At these ultra-high temperatures, the potential exists to use more compressible prealloys, yet still achieve complete homogenization within the final part. The result? New levels of density are possible.  

Also of note’ there’s greater pore rounding with UHTS. This seemingly small feature has been shown to improve:

  • Tensile strength
  • Fatigue strength
  • Impact strength

Elsewhere, researchers are using a souped-up vacuum furnace to achieve these high sintering temperatures. However, sintering in a vacuum furnace is time-intensive and may vaporize any chrome or manganese additions and cause them to precipitate on the furnace’s cold walls.  

To give you a more concrete picture of what UHTS can do, here’s a little experiment on optimizing materials and sintering conditions. The resulting impact on mechanical properties should get you excited:

Experiment: Sintering Structural Powder Metallurgy Components at 2500°

(This data belongs to Horizon Technology. Do not reproduce it for your own purposes.)

The experimental procedure happened in two stages

  1. Alloy optimization and the development of 2500° sintering levels to create “production-level” conditions (rather than prototyping conditions)
  2. Refinement of Stage 1, restricting alloy variants to those that showed the best mechanical properties


Stage 1:  Alloy Optimization

To investigate the effects of alloys on the properties of powder metallurgy steels, we investigated six potential premixes: 

Alloy compositions evaluated

Mix ID

Base Iron

% V

% Cr

% Si

% Ni

% graphite

1

Pure Iron

0.45

1.10

0.70

1.00

0.60

2

FL-3900

0.45

1.10

0.70

1.00

0.60

3

FL-4400

0.45

1.10

0.70

1.00

0.60

4

FL-4400

0.25

1.10

0.70

1.00

0.60

5

FL-4400

0.45

1.10

0.70

1.00

0.85

6

FL-4400

0.45

1.10

0.70

1.00

0.40

7

FL-4400

0

0

0

2.00

0.60

 

  • The 0.30% molybdenum and 0.85% molybdenum base iron materials were prealloyed. 
  • The chromium and silicon additions were added as an iron-silicon-chromium master alloy
  • The vanadium addition was added as a 50% vanadium ferro-vanadium
  • The nickel addition was added as carbonyl nickel 
  • A standard FLN2-4405 material was also evaluated as a reference material (mix #7 in the table). 

From these premixes, we compacted samples at 690 MPa (50 tsi).

Sintering was done both at 2300° in 90% nitrogen and 10% hydrogen atmosphere, and at 2500° in a 50/50 nitrogen-hydrogen atmosphere. All sintering was done with accelerated cooling to promote a sintered-hardened microstructure. 

 

Stage 2:  Evaluation of Optimized Alloying

Using the preliminary results of Stage 1, we condensed the alloy list to those that showed the most promising mechanical properties. 

In this stage, we prepared and evaluated both conventional premixes and premixes intended for warm die compaction. After compaction, sintering occurred at 2300° in 90% nitrogen and 10% hydrogen atmosphere, and at 2500° in 50% argon and 50% hydrogen:

‘Most Promising’ Alloy Evaluation

Mix ID

Base Iron

% V

% Cr

% Si

% Ni

% graphite

Type of premix

8

FL-4400

0.25

1.10

0.30

1.00

0.40

Warm Die 

9

FL-4400

0.25

1.10

0.30

1.00

0.65

 

Post-sintering treatment of the UHTS samples included heat treatment at about 1635° in a 25% nitrogen/75% hydrogen atmosphere, oil quenching, and tempering just under 400°. The rationale was to evaluate the mechanical properties of a 100% tempered martensitic microstructure. 

Results:

Stage 1: Alloy Optimization

First, we determined the compressibility of the various premixes by compacting samples with room-temperature dies.

Compressibility Data for the Various Premixes

Compaction Pressure, MPa (tsi)


Premix 1


Premix 2


Premix 3


Premix 4


Premix 5


Premix 6


Premix 7

410 (30)

6.72

6.67

6.67

6.68

6.66

6.68

6.78

550 (40)

6.94

6.90

6.90

6.94

6.89

6.93

7.04

690 (50) 

7.06

7.04

7.04

7.07

7.01

7.08

7.17

830 (60)

7.16

7.12

7.12

7.15

7.09

7.17

7.26

 

As you see above, Premix 7 (the standard FLN2-4405) exhibited the highest compressibility. Premixes containing the additives showed compressibility decreases up to 0.20 g/cm³ compared to the standard FLN2-4405.

Other notes:

  • Decreasing the vanadium in the FL-4400 premixes prompted a minor improvement in compressibility (see Premix 3 vs. Premix 4).
  • As expected, increasing the graphite (Premix 4 vs. Premix 5) also decreased compressibility.
  • Using a high-compressibility pure iron vs. a prealloyed 0.85% molybdenum steel caused a minor improvement in green density. 
  • Premix 6, which was intended as a potentially carburizing grade, showed minor improvement in compressibility vs. Premix 5. 
  • Using a 0.30% molybdenum prealloy base iron resulted in reduced compressibility vs. the 0.85% molybdenum base iron.    

Mechanical property data from the initial sintering trials is below. The data represents compaction in room-temperature dies and at 690 MPa (50 tsi) pressure:  

2300° F Sintering in 90% Nitrogen and 10% Hydrogen

Premix 

ID

Green 

Density, 

g/cm°

As Sinter Hardened

Tempered at 400° F

Sintered Density, g/cm³

TRS, MPa,
(ksi) 

Hardness, HRA (HRC)

Tempered Density, g/cm³

TRS, MPa,
(ksi)

Hardness, HRA (HRC)

Mix 1

7.08

7.05

1393 (202)

60 (20)

7.05

1475 (214)

61 (21)

Mix 2

7.04

7.01

1627 (236)

67 (33)

7.01

1813 (263)

67 (33)

Mix 3

7.07

7.01

1758 (255)

69 (37)

7.02

1889 (274)

68 (35)

Mix 4

7.09

7.05

1944 (282)

69 (37)

7.05

2027 (294)

70 (39)

Mix 5

7.02

6.92

1000 (145)

75 (49)

6.94

1606 (233)

72 (43)

Mix 6

7.09

7.08

1606 (233)

62 (23)

7.06

1648 (239)

62 (23)

Mix 7

7.13

7.25

1655 (240)

61 (21)

7.26

1634 (237)

60 (20)

 

Mechanical Properties of Premixes Sintered at 2500° F in 50% Nitrogen/50% Hydrogen, Accelerated Cooled and Tempered at 400° F

Premix ID

Sintered Density, g/cm³

TRS, MPa,
(ksi)

HRA (HRC)

1

7.05

1523 (221)

62 (23)

2

7.06

1909 (277)

64 (27)

3

7.05

2055 (298)

68 (35)

4

7.08

2206 (320)

69 (37)

5

7.00

1675 (243)

74 (47)

6

7.10

1640 (238)

61 (21)

7

7.27

1958 (284)

65 (29)

 

As you can see, the green-to-sintered density didn’t change dramatically for either environment. 

Meanwhile, TRS values were affected by the accelerated cooling (sinter hardening). Typical for sinter-hardened materials, tempering improved the overall TRS. 

Other notes:

  • Sintering at 2500° and tempering did show some rather dramatic increases in TRS values, in particular for Premix 4 (reduced vanadium content).
  • Premix 5 was prepared with 0.85% added graphite and was originally intended as a sinter-hardening material.
  • Conversely, Premix 6 was prepared with 0.4% graphite and was intended as a carburizing grade.
  • Premix 1 (pure iron base) had the lowest overall TRS.
  • Premix 3 (0.30% molybdenum prealloy base iron) performed lower than the equivalent FL-4400 base materials. 
  • Premix 4 (0.25% vanadium) gave the highest overall TR strength, regardless of sintering condition.

Remember that initial sintering happened in a nitrogen-hydrogen atmosphere. To partially explain the less-than-expected densities and strengths, we performed residual nitrogen analysis on these materials. All premixes with vanadium showed a dramatic increase in the sintered nitrogen content. The only exception was Premix 5 (0.85% added graphite) -- this showed a minimal increase in nitrogen, from nominally zero to 0.03%.

You could speculate that the high carbon resulted in the formation of vanadium carbides, thus blocking the formation of vanadium nitrides.

Powder Metallurgy Components - Vanadium Chart

(Nitrogen content vs. vanadium content of the premixes)

Metallographic analysis of these materials showed some interesting trends. The 2500° sintering showed significantly greater pore rounding coupled with reduced porosity that outlined the prior particle boundaries. Etched analysis showed greater martensitic transformation as a result of greater alloying diffusion into the iron.  

Powder Metallurgy Components - premix 2

Powder Metallurgy Components - premix 2 at 2500

Premix 2 sintered at 2300° F

Premix 2 sintered at 2500° F

 

Powder Metallurgy Components - premix 4 2300

Powder Metallurgy Components premix 4 at 2500°

Premix 4 sintered at 2300° F

Premix 4 sintered at 2500° F

  
Complementing the metallographic analyses you see above, we performed SEM EDX analysis (scanning electron microscopy and energy dispersive x-ray) on selected samples from Stage 1.  We emphasized alloy maps to determine the dispersion of additives -- particularly, the relative relationship of vanadium, chromium, and nitrogen content.

The results are below:

sintering Powder Metallurgy Components EDAX Maps with nitrogen

(EDX mapping of alloying elements of Premix 1, sintered at 2300° F in 90% nitrogen / 10% hydrogen)

In general, alloy dispersion appeared to be uniform throughout the sample, and there seems to be good correlation between the vanadium, chromium, and nitrogen. This metallographic result, coupled with the nitrogen analysis in the earlier vanadium line graph, suggests that some vanadium and chromium combined with the nitrogen present. In the EDX map, it appears the nitriding occurred during the heat-up stage of sintering.

The conclusion? The vanadium and chromium weren’t fully contributing to the strength of the alloy. 

The high nitrogen pickup and uninspiring mechanical properties prompted a change in the sintering atmosphere for the 2500° experiment. We modified the atmosphere to 50% hydrogen and 50% argon to prevent nitriding of the vanadium and chromium.

So, where did it take the performance?

Stage 2 Results: Optimized Alloys & Processing

In this stage, we sintered the test subjects at 2300° F in 10% hydrogen and 90% nitrogen and at 2500° F in 50% hydrogen and 50% argon. The source of chromium and silicon were modified to be high-carbon ferro-chrome and a 75% silicon ferro-silicon. The transverse rupture strength of the alloys follows:.  

Results of TRS for ‘Most Promising’ Alloys

Premix ID

Green density, g/cm³

Sintered at 2300° F

Sintered 2500° F

Tempered Density, g/cm³

Tempered TRS, MPa  (ksi)

Apparent Hardness, HRA

Tempered Density, g/cm³

Tempered TRS, MPa  (ksi)i

Apparent Hardness, HRA

8

7.27

7.17

2102 (305)

66

7.14

2412 (350)

67

9

7.24

7.10

1558 (226)

72

7.03

1054 (153)

73

 

Tensile Test Results of Same Premixes, Tempered at 400° F



Premix ID

Sintered at 2300° F

Sintered 2500° F

Yield strength, MPa, (psi)

Tensile Strength, MPa, (psi)

Elongation

Yield strength, MPa, (psi)

Tensile Strength, MPa, (psi)

Elongation

8

862 (125,000)

1117 (162,000)

1.5

917 (133,000)

1289 (187,000)

1.7

9

875 (127,000)

917 (133,000)

<1.0

503 (73,000)

752 (109,000)

<1.0

 

Effect of Quenching & Tempering on 2500° F Sintered Materials

Premix ID

Green density, g/cm³

Sintered density, g/cm³

Yield strength, MPa, (psi)

Tensile Strength, MPa, (psi)

Elongation, %

8

7.27

7.13

1262 (183,000)

1469 (213,000)

1.7

9

7.24

7.05

1117 (162,000)

1151 (167,000)

<1.0

 

Carbon, Oxygen, and Nitrogen Analysis of 2500° F (50% Hydrogen, 50% Argon)

Premix ID

Carbon, %

Oxygen, %

Nitrogen, %

8

0.50

0.12

0.006

9

0.75

0.16

0.002

 

Again, at 2300°, the vanadium and chromium tended to combine with the nitrogen and form nitrides. As expected, the nitrogen content of the 50/50 hydrogen-argon sintering showed no nitrogen pickup. (See the table directly above.) 

At 2500°, the tensile properties of premixes 8 and 4 were exceptional in the sinter-hardened condition. However, when these materials received a secondary quench-and-temper treatment in a 25% nitrogen/75% hydrogen atmosphere, the mechanical properties increased. This secondary heat treatment gave warm-die-compacted 0.25% vanadium material a tensile strength of 1469 MPa (213,00 psi) as well as 1.7% tensile elongation.

This combination of yield strength, tensile strength, and elongation exists nowhere in the MPIF Standard 35.

sintering Powder Metallurgy Components EDAX Maps half Nitro

(SEM EDX mapping of alloying elements of Premix 4, sintered at 2500° F in 50% argon/50% hydrogen)

The images above show that the alloying additions were uniformly dispersed throughout the microstructure. However, the absence of nitrogen in the furnace prevented the formation of vanadium and chromium nitrides like we saw previously.

These elements can now contribute to the hardenability and strength of the material, in both the sintered-hardened and quench-and-tempered conditions. 

Impact of Ultra-High-Temperature Sintering on Structural Components

Sintering at 2500 °F offers the option of unique alloys that promise exceptional mechanical properties:

  • Strength
  • Wear resistance
  • Toughness

The ability to add unique alloys as elemental additions minimizes the production hurdles typical in the development of new powder metal materials. In this study, we developed an optimal alloy that used a FL-4400 base, premixed for:

  • 0.25% vanadium
  • 0.6% silicon
  • 1% chromium
  • 1% nickel  

This alloy, when properly sintered, developed tensile strengths exceeding 1450 MPa (210,000 psi) with about 1.7% tensile elongation at a sintered density of about 7.15 g/cm³.  This combination of strength and ductility will open opportunities for new applications currently not served by powder metallurgy. 

The compressibility of this new alloy was about 0.10 g/cm³ lower than a reference FLN2-4405 material. The additional elemental alloying triggered this decrease. While we predicted UHTS would prompt sinter densification; this didn’t happen. (We’re still investigating why there was no shrinkage.)

Despite this obvious drawback, the UHTS alloy staying close to die size in the sintered condition minimizes one potential limitation of sintering -- ability to maintain tight dimensional control.

The sintering atmosphere was extremely important to the mechanical properties the alloy reached. Although the presence of vanadium nitrides (gained in a nitrogen-rich atmosphere) is desirable from a strength perspective, the presence of large particles is not optimal for strength or ductility.  

In a nitrogen-free atmosphere, ferro-vanadium can completely diffuse into the base iron and contribute to strength and hardenability. However, this makes dispersion strengthening unavailable.

How do we solve this Catch-22? The separate quench-and-temper treatment. By combining UHTS temperatures with the 25% nitrogen/75% hydrogen austenitizing step, your part receives more strength while maintaining significant elongation.

In addition to the use of vanadium, small additions of silicon, chrome, and nickel will take mechanical properties even further into the future!

To learn more about the possibilities of PM, or your own application, click below:

Engineer's Guide to Sintering Download

Topics: Powdered Metallurgy, sintering, electrification, machining


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